CDMA (code-division, multiple access) requires that all signals received at a base station from handsets (e.g., cell phone) have about the same power level. When signals of differing power levels are received by the base station, undesirable interference is introduced into the system. In this regard, the handset utilizes different amounts of power to transmit signals to the base station depending on the distance from the base station. For example, when a handset is close to a base station, the handset uses less power to transmit the signal. Similarly, when a handset is far from a base station, the handset uses more power to transmit the signal.
Consequently, an important component in handsets, especially CDMA handsets, is a power amplifier integrated circuit that provides power management. A power amplifier is most efficient when the amplifier is operated at the maximum voltage. For example, a typical power amplifier can generate a maximum of about 26 dBm of power or about 0.4 watts (400 mW). However, most of the time the handset operates with a mid-range power (e.g., between 10 to 16 dBm of power or about 40 mW) because the handset may be closer to a base station.
One approach to improve power amplifier efficiency is to vary the supply voltage as a function of output power. For example, a DC to DC converter may be utilized to down convert from a first voltage to a second voltage (V_1 to V_2). One disadvantage of this approach is that this transformer-based approach utilizes magnetic components that are relatively large. Consequently, this approach is not conducive for space-efficient designs, which is the current trend. For example, as the handsets decrease in size or as more functions are integrated into the handsets, the space that is allocated to power management and control is rapidly decreasing.
Another approach to employ two separate power amplifiers, one power amplifier for high power and another power amplifier for low power. Unfortunately, when the high power amplifier is on, and the low power amplifier is off, the load for the low power amplifier acts as parasitic impedance that dissipates power, thereby decreasing battery life, which translates into less talk time for cellular telephones.
Based on the foregoing, there remains a need for a mechanism to improve power amplifier efficiency by varying the load impedance as a function of output power that overcomes the disadvantages set forth previously.